In general, a liquid developer for electrostatic photography (electrophotography) is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, or phthalocyanine blue, a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, or synthetic rubber, in a liquid having a high electric insulating property and a low dielectric constant such as a petroleum aliphatic hydrocarbon, and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, or a vinyl pyrrolidone-containing polymer, to the resulting dispersion.
In such a liquid developer, the resin is dispersed in the form of insoluble latex grains having a grain diameter of from several nm to several hundred nm. In a conventional liquid developer, however, the soluble dispersion-stabilizing resin and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, so that the soluble dispersion-stabilizing resin and the polarity-controlling agent become freely diffused in the liquid carrier with ease. Accordingly, there is a fault that when the liquid developer is stored for a long period of time or repeatedly used, the dispersion-stabilizing resin is split off from the insoluble latex grains, thereby the latex grains are precipitated, aggregated and accumulated, and the polarity thereof becomes indistinct. Also, since the latex grains once aggregated or accumulated are reluctant to re-disperse, the latex grains remain everywhere in a developing machine attached thereto, which results in causing stains of images formed and malfunction of the developing machine, such as clogging of a liquid feed pump.
In order to overcome such defects, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex grains is disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed therein is still insufficient although the dispersion stability of the grains to the spontaneous precipitation may be improved to some extent. When the liquid developer disclosed in U.S. Pat. No. 3,990,980 is actually used in a developing apparatus, these are some defects that the toner attached to parts of the developing apparatus is solidified in the form of coating, and the toner grains thus solidified are reluctant to re-disperse and are insufficient in re-dispersion stability for practical use, which causes the malfunction of the apparatus and staining of duplicated images.
In the method of producing resin grains described in the above described U.S. Pat. No. 3,990,980, there is a very severe restriction in the combination of a dispersion stabilizer being used and monomer(s) being insolubilized for producing monodispersed latex grains having a narrow grain size distribution. Mostly, the resin grains produced by the above described method are grains of a broad grain size distribution containing a large amount of coarse grains or poly-dispersed grains having two or more different mean grain sizes. In the above described method, it is difficult to obtain mono-dispersed resin grains having a narrow grain size distribution and having a desired mean grain size, and the method often results in forming large grains having a mean grain size of 1 .mu.m or larger or very fine grains having a mean grain size of 0.1 .mu.m or less. Furthermore, there is also a problem that the dispersion stabilizer being used must be prepared by an extremely complicated process requiring a long reaction time.
Further, for overcoming the above described defects, a method of improving the dispersibility, re-dispersibility and storage stability of resin grains by means of forming insoluble dispersed resin grains by copolymerizing a monomer being insolubilized and a monomer containing a long chain alkyl moiety or a monomer containing two or more polar moieties is disclosed in JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Moreover, a method of improving the dispersibility, re-dispersibility and storage stability of resin grains by means of forming insoluble dispersed resin grains by copolymerizing a monomer being insolubilized and a monomer containing a long chain alkyl moiety in the presence of a polymer utilizing a difunctional monomer or a polymer utilizing a macro-molecular reaction is disclosed in JP-A-60-185963 and JP-A-61-63855.
Furthermore, a method of improving the dispersibility, re-dispersibility and storage stability of resin grains by means of forming insoluble dispersed resin grains by copolymerizing a monomer being insolubilized and a monomer containing two or more polar moieties in the presence of a polymer utilizing a difunctional monomer or a polymer utilizing a macro-molecular reaction is disclosed in JP-A-62-166362 and JP-A-63-66567.
On the other hand, recently a method of making a large number of prints such as 5,000 prints or more using a master plate for offset printing utilizing an electrophotographic system has been developed, and as a result of significant improvement of the master plate, it makes possible to obtain more than 10,000 prints of a large size. Also, a noticeable progress has been made in shortening the operation time in an electrophotomechanical system and an improvement of quickening a development-fixing steps in the system has been made.
Further, the rationalization of an electrophotomechanical system has been greatly required and, practically, it has been attempted to prolong a period for maintenance interval of a printing plate making machine. In the attempt, a liquid developer which can be used for a long period of time without being renewed has been required.
The dispersed resin grains produced by the methods as disclosed in the above described JP-A-60-17951, JP-A-60-185963, JP-A-61-63855, JP-A-62-151868, JP-A-62-166326 and JP-A-63-66567 yet show an unsatisfactory performance with respect to the dispersibility and re-dispersibility of the resin grains in the case of increasing the development speed or prolonging the period for maintenance interval, and with respect to the printing durability in the case of shortening the fixing time or using a master plate of a large size such as A-3 size or larger.